Cave fish have evolved to sleep less

Cave dwelling fish have evolved to need much less sleep than their surface cousins in an adaptation that might help them to avoid missing out on rare food.

Cave fish have many adaptations to their dark environments including eyelessness, loss of pigmentation, changed feeding behaviour and metabolic patterns, but this is the first time that researchers have documented genetic changes in sleep patterns. 'Sleep is a characteristic that is easily altered in response to changes in ecological conditions,' said senior scientist Richard Borowsky of New York University, 'we believe that cave fish sleep less, because they need to be awake more in order to detect the arrival of scarce food in the nutrient poor cave environment. If you are asleep when a bit of food floats by, you are out of a meal and out of luck.'

The scientists noticed that while their surface equivalents sleep for much of the night, the cave dwellers patrolled their aquaria and slept as little as 110 minutes during a 24 hour period. Their compatriots slept for 800 minutes on average.

The cave fish sleeping patterns resemble that of humans with sleep disorders. The fish do sleep but only for short periods. Three different cave populations have evolved independently to require less sleep. The Mexican Blind Cave fish studied are an ideal species because they have both eyed surface living members and eyeless cave populations that can be compared.

Lead author of the study, Eric Duboue added, 'No one really knows the function of sleep, but we all know how we feel when we do not get enough. Our discovery of a genetic basis to sleep patterns in cave adapted fish might offer clues to sleep variation in other species and even sleep disorders in humans. The genes controlling sleep are very likely to be the same in fish as in other species.

The next step is to identify the genes responsible by comparing the genome of the cave dwellers to their surface counterparts.

The team also plan to prove their hypothesis about food availability being a key driver to evolutionary changes in sleep patterns by mixing cave and surface fish in a tank over about a month, intermittently feeding them and observing which fish end up thinner.

The study was published in the journal Current Biology this week.

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